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Abstract

We demonstrate that multi-band coherent perfect absorption can be achieved at infrared frequencies by a metasurface in which four-sized columnar metal patches are separated by a dielectric layer in a unit cell. The absorption bandwidth is enhanced by three times compared with single-band absorption while high absorbance is maintained. The coherent perfect absorption is polarization-independent and can be independently modulated at each resonant frequency by tuning the phase difference of two coherent incident beams. Moreover, the resonant frequency is sensitive to the radius of the columnar patch, and thus a wide coherent perfect absorption frequency range can be obtained by adjusting the radius. Through optimizing the structural parameters, nearly perfect absorption at oblique incidence for both TE and TM polarizations are achieved. The optimized metasurface can be used as a beamsplitter at oblique incidence.

Figures (10)

Fig. 1 Schematic of the metasurface which is illuminated by two counter-propagating coherent beams along±z. View of the unit cell where two patterned gold films with thicknessdare separated by a Al2O3 dielectric spacer with thicknessh. Geometrical parameters are the periodpin both x and y-directions, the radius of four column patchesr1,r2,r3,r4.

Fig. 2 (a) Coherent absorption spectra for a metasurface in which the column patches have the same radius in a unti cellr1=r2=r3=r4=r=0.34μm, where the inset depicts the schematic of the proposed subunits. (b) Amplitudes of transmission (Red dotted line) and reflection coefficients (Blue solid line) for a single beam incident normally upon the metasurface. (c) The corresponding phases of transmission and reflection coefficients. (d) Coherent absorption spectra withrvarying from 0.3 to 0.38µm, where arrows indicate the direction of spectral movement.

Fig. 3 Simulated coherent absorption spectra of proposed multi-band metasurface in which the number of columns is different in a unit cell. (a) A column with radiusr1; (b) Two columns with radiusr1,r2; (c) Three columns with radiusr1,r2,r3; (d) Four columns with radiusr1,r2,r3,r4. The four peaks CPA occurs at four frequencies forf1=90THz,f2=96.9THz,f3=104.7THz,f4=114.6THzwith absorption rates of 99.34%, 95.46%, 95.06%, 98.83%, respectively. The bottom four figures are the magnetic field distribution in the center of the dielectric layer at each resonant frequency in which the (e)f1=90THz; (f)f2=96.9THz; (g) f3=104.7THz; (h) f4=114.6THz.

Fig. 5 (a) Coherent absorption of the metasurface as a function of frequency and phase difference, the black and brown dotted lines refer to the absorption at the frequencies of 106.5THz and 112.8THz, respectively. (b) Phase modulation of absorption at f=106.5THz (Black dotted line) andf=112.8THz(Brown dotted line).

Fig. 7 Coherent absorption of the metasurface as a function of frequency and incidence angle for (a) TE polarization, and (b) TM polarization. (c) TE polarization and (d) TM polarization wave incident on the metasurface inx-zplane with an incident angle of θ to z-direction.

Fig. 9 Coherent absorption for TE (a)(c) and TM polarization (b)(d) when the incidence angle is 45°. The optimized parameters are r1=0.38μm,r2=0.35μm,r3=0.36μm,r4=0.37μmfor (a)(b),r1=0.37μm,r2=0.34μm,r3=0.35μm,r4=0.36μmfor (c)(d). The black solid line represents the optimized result while the red dotted line represents the unoptimized result in each graph.

Fig. 10 (a) Coherent absorption of the metasurface as a function of frequency and incidence angle for TM polarization in a wide range of frequencies. (b) The modulation of the point source. The black dotted arrows indicate the absorbed light while the red solid arrows indicate the reserved light.